In an optical communication technology, a distributed feedback laser diode (DFB-LD) is used as a laser diode which oscillates in a single longitudinal mode. The DFB-LD is used for an optical signal transmission at a modulation rate 1.0 G bps or more and a distance of 20 km or more and, especially, the optical signal transmission at a modulation rate 2.5 G bps or more and a distance of 50 km or more.
In order to achieve the single mode oscillation, the DFB-LD generally has a phase-shifter which shifts the phase by Λ/4 in diffraction gratings (see Non-Patent Document 1, for example). Note that the present invention, the magnitude of the phase shift is not shown by “λ” representing the oscillating optical wavelength, but by “Λ” representing twice the distance between the diffraction gratings.
The DFB-LDs in which the magnitude of the phase shift is adjusted are disclosed in, for example, Patent Documents 1-3. Patent Document 1 discloses a DFB-LD in which diffraction gratings having the magnitude of a phase shift of Λ/16 to 3Λ/8 for operating at a stable single axial mode. Patent Document 2 discloses a DFB-LD which has a magnitude of a phase shift of Λ/2 to Λ/n where n=4 to 16, for suppressing wavelength chirping upon modulation. Patent Document 3 discloses a DFB-LD which has two phase-shifting portions each having the magnitude of a phase shift of, for example, Λ/8 for improving emission efficiency in a stable single mode.
[Patent Document 1]
JP Patent Kokai Publication No. JP-A-63-32988
[Patent Document 2]
JP Patent Kokai Publication No. JP-P2003-204114A
[Patent Document 3]
JP Patent Kokai Publication No. JP-P2003-152272A
[Non-Patent Document 1]
    “LASER KOGAKU NYUMON” (Introduction to Laser Engineering), supervised by Fumio Inaba, edited by IEICE (The Institute of Electronics, Information and Communication Engineers), 1997, pp. 172-176